1. Field of the invention
This invention relates generally to a seal assembly for pumps and were particularly, though not exclusively to centrifugal slurry pumps.
2. Discussion of the Related Art
Gland seal assemblies have been used on centrifugal water and slurry pumps for many years. Typical conventional gland assemblies are shown in FIGS. 1, 2 and 3 FIGS. 1, 2 and 3 are partial schematic sectional elevations of conventional gland seal assemblies. Such assemblies generally consist of an outer housing 2 which is normally called the stuffing box, with a cylindrical bore 4, through which passes a rotatable shaft 50 which may for example be, a pump shaft. An end wall 35 is disposed at one end of the bore. Shaft 50 may or may not have a protective sleeve thereon. The annular gap between the bore 4 and the shaft 50 is typically filled with packing 5 which may be in the form of a number of packing rings 8. The packing material can vary depending upon the application but is normally woven from fibres in a square or rectangular section which can be cut into annular rings. Materials are chosen for strength and other lubricants (such as PTFE) may be added to reduce friction.
Packing 5 is normally placed into the housing or stuffing box and compressed from the outside end by a gland 9. Due to the compressibility of the packing 5 this allows the gap between the packing 5 and shaft (or its sleeve) 50 to be adjusted by moving the gland 9 into and out of the annular gap forming a packing space. External bolts (not shown) normally permit this adjustment to be effected. Adjusting the gap between the stationary packing 5 and the rotating shaft 50 allows the leakage from the assembly to be regulated. This leakage may in certain applications be critical to ensure proper cooling of the packing and shaft and to remove the heat generated by friction.
Sealing assemblies of this type for water pumps can utilize the pumped liquid that leaks from the sealing assembly during operation to help cool the assembly. Adequately lubricated assemblies are necessary for a long life.
Slurry pumps offer an additional problem because the slurry being pumped contains particles. These particles cause additional friction and wear to the packing and sleeve. For slurry pumps it is common practice to inject clean sealing water from an external supply into the assembly to reduce these problems and ensure a long life. The water is injected into the assembly via a feed channel 14 to a lantern ring assembly 10. The lantern ring assembly may comprise a lantern ring 11 and a restrictor 12. In FIG. 1 the restrictor 12 is formed of metal and in FIG. 2 it is a non-metallic device. In FIG. 3 there is a lantern ring 11 which is spaced from a neck ring 15 by a packing 8D. The lantern ring and lantern restrictors direct the water introduced via channel 14 into a gap around the shaft or sleeve thereon. This allows water into the critical gap between the packing 5 and the shaft 50 for proper and effective lubrication. Both lantern ring and lantern ring restrictor arrangements allow some sealing water to flow into the pump. This has the desired effect of flushing solids or particles away from the sealing assembly, hence minimising the risk of slurry contamination into the gland.
Such conventional arrangements described above have inherent problems in their design as well as causing operational and maintenance problems for slurry pumps. It is a maintenance requirement that pumps need to be repacked (that is new fresh packings replaced for old worn packings) without the necessity of a major pump strip down. Here the small annulus around the shaft sleeve presents a problem because it is deep and very difficult, if not impossible to look into during repacking. Hence a lot of repacking work is done by feel. Repacking is made more difficult by the lack of access at the back of centrifugal slurry pumps. A further problem is associated with operators who do not always make proper adjustments. Water supply can be variable or even fail leading to wear and failure of the packings and sleeve. Water can also leak around the outside instead of the inside of the diameter of the packings.